The invention relates to a method for the printing of a print fabric.
In form based printing machines preferably operating according to the offset printing principle such as for example in web-fed rotary printing presses and in sheet-fed printing presses inkjet printing devices without form are increasingly used which more preferably serve for the individualization of printing products produced via offset printing with, for example, bar codes, numbering or other markings. Such inkjet printing devices have at least one inkjet printing head which can be designed according to the so-called continuous inkjet principle, the drop-on-demand inkjet principle, the thermal inkjet principle, the bubble inkjet principle or any other inkjet principle. The inkjet printing heads usually have a nozzle row of several nozzles arranged next to one another by way of which the printing ink can be directed at a print fabric to be printed.
Since the maximum printing speed of inkjet printing devices is significantly lower than the maximum printing speed of offset printing devices, in-line printing of a print fabric according to offset printing and according to inkjet printing poses difficulties. To increase the printing speed of inkjet printing devices that can be achieved it is already known from practice to use inkjet printing devices with a multiplicity of inkjet printing heads, namely on the one hand with several inkjet printing heads across the transport direction of the print fabric or the print direction and on the other hand with several inkjet printing heads in transport direction of the print fabric or in printing direction, wherein the multiplicity of inkjet printing heads are arranged next to one another array-like or matrix-like.
The number of inkjet printing heads required across the printing direction is primarily defined by the desired print resolution relative to the given print resolution of the inkjet printing head used and by the desired overall printing width relative to the given printing width of an inkjet printing head. The required number of inkjet printing heads in printing direction is primarily determined by two points, namely firstly in that the desired printing speed is greater than the given printing speed of an inkjet printing head and on the other hand in that several printing inks are to be applied to a print fabric via the inkjet printing device.
Independent of whether an inkjet printing device with several inkjet printing heads arranged array-like or a single inkjet printing head for printing of a print fabric is used, the printing speed that can be achieved can also be increased in that the, or each, inkjet printing head of an inkjet printing device is obliquely aligned or inclined to the transport direction of the print fabric and thus to the printing direction. The inclination results in that the effective distance of the nozzles across the printing direction or transport direction of the print material is reduced and the print resolution across the printing direction can thus be increased. If the printing speed remains unchanged it is then possible to print with a higher area coverage or optical density. Likewise it is also possible to keep the area coverage or optical density constant while increasing the printing speed.
If, to increase the print resolution and/or to increase the printing speed on inkjet printing devices, work is performed with inkjet printing heads inclined to the printing direction or the transport direction of the print fabric, the output data for an image to be printed with the inkjet printing device provided in a preliminary stage of printing has to be converted according to the given geometrical conditions.
With printing methods known from practice this conversion is carried out in the hardware of the inkjet printing heads which however has the disadvantage that this conversion is valid only for a defined inclination, only for a defined drop frequency and only for a defined printing speed. If for instance the printing speed should change it is not possible to react to this as a result of which distortions ultimately impairing the print quality are obtained for the print image to be printed.
Based on this the present invention is based on the problem of creating a new type of method for printing a print fabric. According to the invention, output data more preferably an output data matrix of a print image to be printed with the inkjet printing device is converted into target data, more preferably a target data matrix for controlling the inkjet printing device in real time dependent on a current printing speed, dependent on a current drop frequency of the, or each, inkjet printing head of the inkjet printing device and dependent on a current inclination angle of the, or each, nozzle row of the, or each, inkjet printing head relative to the transport direction of the print fabric prior to transmitting data to the inkjet printing device.
In terms of the method according to the invention it is provided to perform the conversion of the output data to the target data for controlling an inkjet printing device inclined in printing direction independent of the hardware of the inkjet printing heads of the inkjet printing device. The conversion of the output data to the target data according to the invention accordingly takes place prior to the transmission of image information from the preliminary printing stage to the inkjet printing device and thus between the preliminary printing stage and the inkjet printing device. The conversion of the output data to the target data according to the invention takes place in real time wherein the current printing speed, the current drop frequency and the current inclination angle are variable quantities in the conversion of the output data to the target data.
Because of this, the conversion of the output data to the target data can for example be adapted to a changing printing speed so that a high print quality can be guaranteed with the inkjet printing device even with changing printing speeds.
According to a first advantageous further development of the invention the conversion of the output data to the target data is performed by way of a transformation such that an output data matrix is scaled and sheared in printing direction and across the printing direction. A scaling factor for scaling the output data matrix across the printing direction is determined from the current inclination angle, namely from the ratio of the expansion of the print image across the printing direction with inclined inkjet printing device to the expansion of the print image across the printing direction with non-inclined inkjet printing device. A scaling factor for scaling the output data matrix in printing direction is determined from the current printing speed and the current drop frequency. A shear angle for shearing the output data matrix is determined from the current inclination angle.
According to a second alternative advantageous further development of the invention the conversion of the output data to the target data is carried out such that an output data matrix is scanned step-by-step dependent on the current inclination angle, the current printing speed and the current drop frequency wherein, then, when one or several nozzle positions of the inkjet printing device impinge on one pixel in an output data matrix, a corresponding pixel is set in a target data matrix.
Preferred further developments of the invention are obtained from the following description. Exemplary embodiments of the invention are explained in more detail by the drawings.